Litharge, which is a chemical product whose formula is PbO, is used in several industrial sectors and one among the most common methods for producing it is the one which resorts to the calcination of massicot, which is a mixture of lead oxide and metal lead and has the chemical formula PbO+.sup.Pb.
The presently used kilns comprise a plurality of tubes, lapped by the flue gases generated by the combustion, e.g., of methane gas, and so connected as to run along an essentially "S"-shaped trajectory.
Massicot, charged to an end of the "S"-shaped path leaves it (after being heated and oxidated), transformed into litharge, at the opposite end.
The movement of advancement of the material takes place in countercurrent to the flue gases generated by combustion.
The kilns known from the prior art show several drawbacks, the main of which are briefly described in the following.
(a) only a portion of the kiln, and more precisely about half thereof, is active as regards the conversion of massicot into litharge; the residual portion of the kiln performs the function of recovering the thermal energy produced by the kiln, by pre-heating massicot, which will be subsequently transformed into litharge. As a consequence, the dimensions of the kiln are very large relatively to its throughput.
(b) Owing to its large dimensions, the kiln reaches its steady-state operating temperature with poor precision in the various regions; in fact, the value of temperature inside the kiln is influenced by many factors interacting with one another, such as, e.g., the temperature of the combustion chamber, of the burner, the flow rate of the air stream injected into the kiln, and the heat dispersions. Controlling such factors becomes more and more difficult as the dimensions of the kiln increase.
(c) The need for providing at least one combustion chamber and for reducing energy consumption by recovering a portion of the generated heat by pre-heating massicot, causes a further increase in kiln dimensions.
(d) The large surface-area of the external surfaces of the combustion chamber cause considerably heat losses towards the external environment, to which the heat losses causes by the stack add up.
(e) The products of the combustion submit the tubes lapped by the flue gases to considerable stresses of chemical and thermal nature, with frequent interventions for inspection and maintenance being necessary.
(f) Also the lining structure of refractory bricks directly invested by the flames and/or highly corrosive gases, generated by the combustion, requires frequent interventions for inspection and maintenance.
(g) The possible perforation of, or formation of crevices in, one or more tubes, causes the coming of the product contained in the tube into direct contact with the flame, with the consequent dispersion of lead oxide into the surrounding environment, through the stack, with evident ecological damages.